Silicone release compositions, and especially paper release compositions, are widely used as coatings which release pressure-sensitive adhesives for labels, transfer tapes, decorative laminates, and the like. Such silicone products are most commonly sold as dispersions of reactive high molecular weight polysiloxane gums in organic solvents, such as toluene. A crosslinking or curing catalyst is then added to the dispersed low-solids mixture, the coating blend is applied to the substrate which is then passed through an oven to evaporate the carrier solvent and cure the silicones to a relatively non-adherent release surface. This process requires a large thermal energy input in order to properly evaporate the solvents and effect the crosslinking reaction at commercially viable rates.
Rising energy costs coupled with stringent environmental regulation of solvent emissions have made the use of solvent-borne silicone release agents increasingly uneconomical. While solventless or emulsion-borne silicone compositions can solve the environmental problems, high oven temperatures and expensive energy usage are still required for their proper application.
Radiation-curable silicone release compositions successfully address both the energy and environmental problems inherent in the use of traditional solvent-dispersed silicones. For example, an ultraviolet (UV) radiation-curable solventless silicone release composition eliminates the need for energy-intensive ovens as well as expensive solvent recovery apparatus. Such materials are not unknown; considerable literature in the field of UV-curable silicone compositions has been noted in recent years, although commercial introduction of such products has not yet occurred. Applicant's copending application Ser. No. 63,648 filed Aug. 3, 1979, and which is hereby incorporated by reference, describes epoxy-functional silicone compositions which are curable upon exposure to ultraviolet radiation and which utilize certain bis-aryl halonium salts as photoinitiators.
Several other UV-cure silicone systems have been described. Patents issued to R. V. Viventi (U.S. Pat. No. 3,816,282 issued June 6, 1974), Bokerman et al. (U.S. Pat. No. 4,052,529 issued Oct. 4, 1977), and Colquhoun et al. (U.S. Pat. No. 4,070,526 issued Jan. 25, 1978) are representative of those compositions wherein omega-mercaptoalkyl substituents attached to polysiloxanes add to vinyl-functional siloxanes in a free-radical process in the presence of certain photosensitizers upon UV irradiation. However, those silicone materials which include mercaptoalkyl functional protoactive substituents also possess an offensive odor (associated with the mercaptan group) which persists in the cured material.
The present invention provides newly developed radiation-curable silicone paper release compositions which do not require scarce or expensive inputs. These novel compositions are comprised of acrylic or methacrylic-dimethylsilicone fluids which will cure via well-known free radical processes when irradiated with UV light in the presence of standard photoinitiators.
Acrylic-functional silicones are themselves not a new concept. R. L. Mercker described polymers prepared from acryloxymethyl and methacryloxymethyl substituted organosilicone compounds in U.S. Pat. No. 2,956,044 which issued Oct. 11, 1960. Merker's syntheses of acryloxymethyl-substituted silicones required chloromethyl-substituted organosilicones as inputs. Such silicones are themselves prepared by halogenation of methyl-substituted silicones or by reaction of halosilanes with Grignard reagents followed by hydrolysis in order to produce the desired polymer. Neither of these processes is well-suited to large scale commercial production, and chloromethyl-substituted silicone polymers and monomers are scarce and expensive materials.
Another synthetic route to acrylic-siloxane compositions is taught by Nordstrom and Zelek (U.S. Pat. No. 3,650,811 which issued Mar. 3, 1972). The Nordstrom et al. synthesis involves the reaction of omega-hydroxyalkylacrylates or methacrylates with silanol-containing silicones in the presence of condensation catalysts such as tetraisopropyltitanate. In practice, the resulting acryloxy-substituted polymers are severely limited since the reactive acrylic groups are confined to the chainstopper position (i.e. at the ends of the linear polymer molecules). The lack of reactive sites on the polymer chains causes the free-radical crosslinking reaction between the acrylic moieties to proceed very slowly, rendering such compositions impractical for high-speed processing operations common in the paper converting industry.
U.S. Pat. No. 4,201,808--Cully et al, discloses paper release compositions curable by radiation consisting of acryloxy-group-containing silicone fluids plus polyacrylic-crosslinking agents and photosensitizers as needed. Although the Cully et al. disclosure is related to the compositions of the present invention, there are significant differences between the materials described by Cully et al. and those disclosed in the present application. Most important, the Cully et al. disclosure specifies that their coating composition must consist of two parts: the acryloxy-functional silicone fluid plus at least 10% (with 50% being preferred) of a polyacrylate monomer such as trimethylolpropane triacrylate (i.e. a non-silicone reactive diluent). The coating compositions of the present invention do not require reactive diluents or crosslinkers while providing useful rates of cure. Nor are such materials required for good release properties. Furthermore, the compositions of the present invention provide useful release characteristics yet consist solely of an acrylic-functional silicone fluid with photosensitizers as needed. Additionally, although the Cully et al. disclosure does not specify any particular preferred mode of manufacture of the acrylic-siloxane fluids described therein, the examples provided by Cully et al. appear to suggest that hydrosilation addition of allyl(meth)acrylate to hydride fluids is the synthetic route utilized. Whereas, on the other hand, a significant feature of the present disclosure is the production of acrylic-functional silicones via consecutive addition of (meth)allylchloride and acrylic acid to hydride fluids as described herein below.
The compositions of the present invention herein described do not suffer from any of the inherent disadvantages discussed above. All of the inputs for synthesis of these UV-curable paper release compositions are inexpensive and readily available. The synthesis itself is an easy stepwise procedure which can be performed in a single reaction vessel if desired.
It is therefore an object of the present invention to provide novel acrylic functional silicone compositions and copolymers which are capable of being crosslinked upon exposure to ultraviolet radiation.
It is another object to provide acrylic-functional polyorganosiloxanes and copolymers of such siloxanes by a process comprising the steps of adding allylchloride and acrylic acid to hydride-containing siloxanes.
It is another object to provide processes for the synthesis of acrylic functional silicones for use in release coating applications.
It is another object to provide ultraviolet light curable acrylic functional silicone release coating compositions and methods for use.
These and other objects will become apparent to those skilled in the art upon consideration of the following description, examples and claims.